Forgive and forget? Easier said than done. It seemed for a while that Robert Pattinson and Kristen Stewart's relationship was back on track after the actress' cheating scandal last summer with her married Snow White and the Huntsman director, Rupert Sanders, but in truth, Pattinson, 27, never really got over the incident. Sources tell Us Weekly the Twilight actor had a difficult time moving on after Stewart's fling -- and he wasn't shy about letting her know.

"He really put her through the ringer," one insider tells Us. "Ultimately, it was like, 'Either we move past this or we don't.' It wasn't good for either of them."

Indeed, as the new issue of Us Weekly reveals, the Cosmopolis star repeatedly used Stewart's tryst against her in arguments between the two, which happened with increasing frequency toward the end of their relationship. "Rob always brings up the cheating scandal to Kristen," a second source tells Us. "He hangs it over her head."

Time will tell whether the couple's recent split is a temporary break or a permanent breakup, but for now, at least, they're living separate lives. "They're both good people," the first source tells Us. "It was just not the right thing for either of them right now."

For more on what went wrong between the Twilight costars -- including details on their blowup fight on Pattinson's birthday and why this separation might stick -- pick up the new issue of Us Weekly, on stands Friday.